An inrush current is a transient condition where the initial current flowing from a power supply to a newly connected load is much higher than the steady state condition. An inrush current of many times the steady state current rushes into the load to charge the capacitive part of the load before the current settles down to be dependent on the resistive part of the load. Inrush currents are undesirable for a number of reasons, including a drop in supply voltage to other loads connected to the power supply. In addition protection circuits may misinterpret the inrush current as a fault and trip the load. Also, in explosive atmospheres, an inrush current may cause sparking potentially resulting in an explosion.
A number of current limiting circuits are available for incorporation into capacitive loads and which use a semiconductor device such as a FET in the load path between the power supply and load. A sensor resistor also in the load path may be used to determine the load current and reduce the conductance of the semiconductor device in response to detecting an inrush current. Alternatively, the conductance of the FET may be slowly increased after the load is connected, so that the initial inrush current is again limited by the initial low conductance of the FET.
Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated relative to other elements to help to improve understanding of embodiments of the present invention.